A simplified version of a recipe that first appeared on The Perfect Pantry, from Giada DeLaurentiis' Everyday Italian. Serves 8.
3 Tbsp olive oil
1 large fennel bulb, trimmed and thinly sliced (a mandoline works well for this, if you have one)
1 large onion, chopped
A pinch of kosher salt
4 large garlic cloves, finely chopped
1/2 tsp dried crushed red pepper flakes, or more to taste
1 large squirt of tomato paste from a tube (2 Tbsp, more or less)
26 oz canned chopped tomatoes (I use POMI), or fresh chopped tomatoes with juice
1-1/2 cups dry white wine
3 cups homemade chicken stock or store-bought low-sodium chicken broth (I use Swanson 99%)
2 8-oz bottles clam juice
1 bay leaf
1 lb littleneck clams, scrubbed
1 lb mussels, scrubbed and debearded
1-1/2 lb raw "large" shrimp (26-30 size), peeled and deveined
1/2 lb cod loin or scrod or halibut, cut into 2-inch chunks
1 lb salmon (skinless fillet), cut into 2-inch chunks
Heat the oil in a large (8-quart or larger) nonreactive (stainless steel or enameled cast iron) stockpot over medium heat. Add the fennel, onion and salt, and sauté until the onion is translucent, about 5 minutes. Add the garlic and red pepper flakes, and sauté 2 minutes. Stir in the tomato paste, canned tomato with the juices, wine, chicken stock, clam juice and bay leaf. Cover, and raise the heat just until the soup comes to a low boil. Reduce the heat to medium-low. Cover and simmer until the flavors blend, about 30 minutes. (Can be made ahead up to this point; refrigerate or freeze.)
Add the clams, and cook for 15 minutes. Then, add the mussels, shrimp and fish. Simmer gently until the fish and shrimp are just cooked through, and the clams and mussels are open, about 5 minutes (discard any that do not open). Season the soup to taste, with more salt and red pepper flakes. Serve hot.